Certain papermaking systems contain paper pulps bleached or brightened using sodium dithionite, also known as sodium hydrosulfite. The resultant bleached or brightened pulps or mechanical fibers are commonly used in the newsprint and publication grade market segment as well as other paper market segments. These pulps or mechanical fibers can contain significant levels of sulfite residuals in the papermaking wet end which presents a challenge to the papermaking process. Other sources of sulfite include shared white water systems and deliberate addition of sulfite to eliminate traces of chlorine. One problem associated with elevated sulfite levels is that synthetic strength resins based upon glyoxalated poly (acrylamide) have limited effectiveness due to the action of sulfite ion on the resin decreases or eliminates its effectiveness.
In commercial practice, glyoxalated poly (acrylate) resins are prepared by reacting glyoxal with a copolymer of acrylamide and a small amount of cationic comonomer, typically diallyldimethylammonium chloride (DADMAC), to obtain mildly cationic resins. Such resins are described in U.S. Pat. Nos. 3,556,933, 4,605,702 and 5,723,022, the disclosures of which are incorporated herein by reference.
It is well known that glyoxalated resins lose their effectiveness to impart strength to paper in sulfite-containing environments. Sulfite ions are reactive towards the gem-hydroxyl functionalities present in these resins
This problem is recognized in the papermaking art, and was clearly set forth by C. E. Farley in a TAPPI Monograph on Wet Strength Resins and their Application (L. L. Chan, editor, 1994, ISBN 0-89852-060-6, Chapter 3 “Glyoxalated polyacrylamide resin”). The following quote is believed to represent the generally accepted opinion of the effect of sulfite on glyoxalated polyacrylamide resins:                “The resin reacts with sulfite and bisulfite ions present in the paper machine wet end. The anionic bisulfite adduct which forms can offset a portion or all of the cationic charge on the resin, and efficiency is lost due to reduced resin retention in the paper. The presence of sulfites in the paper machine wet end is due to either bleaching (hydrosulfite) carryover or addition of antichlor. Where sulfite levels are controlled at about 2 ppm or less, glyoxalated PAM efficiency is not affected.”        
In order to solve the problem of sulfites' deleterious effect on strength resins, oxidants have been added to the paper machine wet end, or highly cationic resins have been added to complex with the now anionic glyoxalated polyacrylamide.
Glyoxalated poly(acrylamide)s and variants thereof have been disclosed in a number of US and other patents.
U.S. Pat. No. 3,556,932 teaches the use of water soluble, ionic glyoxalated vinylamide wet strength resins and paper made therewith. Specifically, U.S. Pat. No. 3,556,932 teaches the use of cationic vinylamide polymers and copolymers of acrylamide and diallyldimethylammonium chloride in 99:1 to 75:25 ratio. U.S. Pat. No. 3,556,933 teaches the use of sulfite ion to enhance the storage stability of the resins of U.S. Pat. No. 3,556,932, and regeneration of such resins using formaldehyde.
U.S. Pat. No. 4,603,176 teaches the use of glyoxalated terpolymers containing a polar non-nucleophilic unit, which does not cause the resin to become water insoluble, as temporary wet strength agent. A cationic fragment is also included in the compositions taught. U.S. Pat. No. 4,605,702 teaches the use of low molecular weight polymers as a basis for temporary wet strength resins. Specifically, U.S. Pat. No. 4,605,702 teaches the use of 1-30 by weight % of a cationic comonomer or mixture of cationic comonomers copolymerizable with acrylamide. U.S. Pat. No. 4,954,538 teaches the use of microparticles consisting of glyoxalated acrylamide containing polymeric material as wet- and dry strength agents for use in paper production. U.S. Pat. No. 5,723,022 teaches compositions of blends of compositions of U.S. Pat. Nos. 3,566,932 and 4,605,702.
The need exists for resins for imparting strength to paper which are relatively resistant to the level of sulfite ions present in the papermaking process.